1. Field of the Invention
The invention relates to a heatable roll, in particular for processing web-like materials, such as paper.
2. Description of the Prior Art
For the processing of web-like materials, such as paper, rolls are known which can be heated from the inside. In a conventional constructional form a heated liquid heat carrier flows through passages or bores which are formed closely beneath the roll surface. The heat carrier thereby gives its thermal energy at least partially up to the bore wall and thus to the roll wall. The roll wall transfers the thermal energy to the web-like material to be processed, for example to dry it.
In this known constructional form for a heatable roll an external means is necessary for heating the heat carrier liquid and keeping it at a predetermined temperature. In many countries, for example Sweden, containers of cast iron are not allowed for closed heat carrier circuits with temperatures above 140.degree. C. Consequently, chilled rolls cannot be heated to temperatures above the specified maximum temperature. On the other hand, high temperatures are desired in the treatment of web-like materials. It is for example perfectly usual in Germany to employ roll temperatures of 200.degree. C. and more.
Furthermore, heatable rolls are known in which the roll jacket rotates about a stationary core. The stationary core is equipped with one or more induction coils. In operation, via the induction coils eddy currents are induced in the rotating roll jacket which heat up the latter by ohmic heat.
Such a constructional form of a heatable roll has the disadvantage that it is extremely sensitive because in a practical embodiment, for example for use in a paper calender, a roll tube has to be supported in cantilever manner over a span of over 8 m above a coil core of the same length. Shocks, which cannot be avoided in practical operation of paper machine glazing rollers, and unbalances, which as a rule occur to varying extents in heatable rolls, necessarily lead to vibrations which result in contacts between rotating and stationary parts. This leads necessarily to damage of the parts concerned. Premature wear or even total breakdown would be the result.
Finally, it should be mentioned that heatable chilled rolls having an external supply of heat carrier liquid come under further safety regulations in countries such as Sweden with regard to the volume content of heat carrier liquid. These safety regulations are intended to reduce the danger originating from the escaping heat carrier liquid on breakage of such rolls.